The Absolute Threshold test identifies pixels with brightness temperature or radiance so high that the pixel must contain a fire. A potential fire pixel is confirmed as a true fire pixel if S7 brightness temperature is higher than 360 K by day or 320 K by night.

In addition, signals from the S5 and S6 channels should essentially be zero over ambient temperature surfaces at night whereas fires will emit significantly at this wavelength. As a consequence, the signal in this channel is used to detect night-time fires that might be missed by the tests based on fire signals in the S7 and S8 channel signals alone. This is especially the case since the pixel size of these SLSTR optical channels is 500 m, as opposed to 1 km for the thermal channels, so any fire will comprise a higher proportion of the S5 and S6 pixel area. An additional absolute threshold test is therefore used to make use of this capability.